FIG. 1 of the appended drawings illustrates a fan of that type which is constructed in accordance with the prior art. The fan which is generally designated 1 comprises a plurality of external blades 2 which extend from a central hub 3 of cup-like form. The hub has an annular side wall 3a and a bottom wall 3b. The internal ribs or blades 4 of the hub have a longitudinal portion 4a which extends from the side wall 3a of the hub, and a radial portion 4b which extends from and along the bottom wall 3b of that hub. See also FIG. 3 in which such an internal rib according to the prior art is shown.
The internal ribs 4 extend between the side wall 3a of the hub 3 and a central projection 5 of the bottom wall 3b thereof which can be used for keying the shaft of the motor (not illustrated).
As can be seen more clearly in FIG. 3, the profile of each internal rib 4 according to the prior art is substantially L-shape: the longitudinal portion 4a of the blade has a profile approximately parallel with the side wall 3a, except at one end where it is “radial” and concave in order to connect the two portions of the blade, and at the other end in which it is convex arcuate. The extent of the longitudinal portion 4a in the direction transverse to the axis A-A (FIG. 3) of the hub 3 is clearly more pronounced than the extent of the radial portion 4b in the direction substantially parallel with that axis.
In the fan according to the prior art described above, the formation of the internal ribs 4 of the hub 3 is such that the fan has high losses owing to the ventilator effect: the longitudinal portion 4a of the blades 4 brings about the application of a substantial resistance torque with respect to the electric motor. Furthermore, whilst the efficiency of ventilation of the motor may be limited, the additional load owing to the losses is “perceived” by the electric motor as an additional resistance torque.